The present invention relates to a method and an apparatus for controlling the operation of a synchronous motor or more in particular to a method and an apparatus for controlling the operation of a synchronous motor in which the synchronous motor is started with a starting power supply and run in steady state with a steadying power supply.
In the case where a multiplicity of synchronous motors in parallel are operated as a group at the same time, a conventional method so far suggested uses a single inverter or cyclo-converter or other current-type static power converter as a driving power supply for the dual purpose of starting and steady operation.
Such a method, in spite of its advantage that the motor can be started substantially from the zero frequency, has the shortcoming of a high impedance as viewed from the synchronous motor side, making it unsuitable for group operation of a multiplicity of motors from the viewpoint of its stability or especially prevention of hunting.
Since the power converter is used for both starting and steady operation, its power capacity depends on the power required for acceleration. When a large number of motors is involved, therefore, there occurs an uneconomically great difference between the total power capacity and the power capacity required for steady operation.
In view of this, an economical motor driving system is desired whereby the synchronous motors can be started from substantially zero frequency, and which enables a multiplicity of synchronous motors in parallel to be operated with high stability and which has a power capacity substantially corresponding to the power required for steady operation.
One method to achieve the above-described system may be to start and accelerate the synchronous motor by the use of a starting power supply such as a static inverter or other power converter and to effect the steady operation thereof by the use of a steadying power supply. This construction, however, poses a problem of how the starting power supply should be switched to the steadying power supply. In one of the suggested methods for the synchronizing transfer, after accelerating the motor temporarily up to a speed higher than the steady frequency by the starting power supply, the starting power supply is cut off, so that the synchronous motor is naturally reduced in speed. During this natural deceleration, the synchronous motor is connected to the steadying power supply by detecting a synchronization between the steadying power supply and the synchronous motor. Even though this synchronizing system has the advantage of simplicity in control operation, the relation between the frequency accuracy for synchronization and the condition for synchronization, that is, the coincidence in voltage, frequency and phase between the two power supplies is determined by the moment of inertia of the mechanical system including the motor and load as well as by the motor revolutions, resulting in a large value of the moment of inertia. In the case where a high accuracy in frequency is required, therefore, the fact that there is only one chance of synchronization during the natural deceleration of the motor makes synchronization very difficult, often causing a synchronization failure. Once synchronization fails, it is necessary to wait until the synchronous motor stops and to repeat the above-mentioned process for synchronization by accelerating the motor with the starting power supply, leading to the disadvantage of a long time required for final successful synchronization.
Further, in the conventional control system using the current-type power converter as a starting power supply, the synchronous motor is controlled by mechanically detecting a rotational position thereof by means of mechanical parts which often fail and are short in life. For this reason, a control system is desired whereby the synchronous motor is capable of being controlled without any mechanical parts in such a manner as to operate both efficiently and stably.